1. Field of the Invention
The invention relates generally to the field of electromagnetic survey apparatus for subsurface exploration in the Earth. More particularly, the invention relates to structures for detector electrodes and arrays thereof for detection of electromagnetic fields resulting from electromagnetic energy imparted into the Earth.
2. Background Art
Electromagnetic surveying is used for, among other purposes, determining the presence of hydrocarbon bearing structures in the Earth's subsurface. Electromagnetic surveying includes what are called “controlled source” survey techniques. Controlled source electromagnetic surveying techniques include imparting a time varying electric current or a time varying magnetic field into the Earth, when such surveys are conducted on land, or imparting such fields into sediments below the water bottom (sea floor) when such surveys are conducted in a marine environment. The techniques include measuring voltages and/or magnetic fields induced in electrodes, antennas and/or magnetometers disposed at the Earth's surface, in the water or on the water bottom. The voltages and/or magnetic fields are induced by interaction of the electromagnetic field caused by the electric current and/or magnetic field imparted into the Earth's subsurface (through the water bottom in marine surveys) with subsurface Earth formations.
One type of marine controlled source electromagnetic surveying known in the art includes imparting alternating electric current into the sediments below the water bottom by applying current from a source, usually disposed on a survey vessel, to a bipole electrode towed by the survey vessel. The bipole electrode is typically an insulated electrical cable having two electrodes thereon at a selected spacing, sometimes 300 to 1000 meters or more. The alternating current has one or more selected frequencies, typically within a range of about 0.1 to 10 Hz. A plurality of detector electrodes is disposed on the water bottom at spaced apart locations, and the detector electrodes are connected to devices that record the voltages induced across various pairs of such electrodes. Such surveying is known as frequency domain controlled source electromagnetic surveying. Frequency domain EM survey techniques are described, for example in U.S. Pat. No. 7,026,819 issued to Eidesmo et al.
Another technique for controlled source electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying. In transient controlled source electromagnetic surveying, electric current is imparted into the Earth's subsurface using electrodes on a cable similar to those explained above as used for frequency domain surveying. The electric current may be direct current (DC). At a selected time or times, the electric current is switched, and induced voltages are measured, typically with respect to time over a selected time interval, using electrodes disposed on the water bottom or towed in the water as previously explained with reference to frequency domain surveying. Structure and composition of the Earth's subsurface are inferred by the time distribution of the induced voltages. Transient electromagnetic surveying techniques are described, for example, in U.S. Pat. No. 6,914,433 issued to Wright et al.
Some types of marine electromagnetic survey systems use pairs of electrodes disposed at spaced apart locations along one or more streamer cables for detecting electric field components of electromagnetic fields in the subsurface. One type of electrode for use in a towed marine streamer is described in U.S. Pat. No. 7,446,535 issued to Tenghamn et al. and assigned to the assignee of the present invention. The electrode pairs are coupled across input terminals or respective preamplifiers, the output of which is communicated to signal processing and recording devices. Another example electrode-type electromagnetic receiver system is disclosed in U.S. Patent Application Publication No. 2009/001987 filed by Davidsson, the underlying patent application for which is assigned to the assignee of the present invention. The system disclosed in the '987 publication includes a plurality of sensor modules each having a single electrode thereon, and a common reference electrode extending between all of the sensor modules. Voltages impressed between the common reference electrode and each of the sensor module electrodes are measured in response to the electric and/or magnetic fields imparted into the subsurface formations below the water bottom.
In marine electromagnetic survey systems such as the foregoing that use electrodes to detect electric field components of electromagnetic fields from the subsurface, it is desirable to be able to determine the response characteristics of the electrodes over time, as such response characteristics may change. It is particularly desirable to be able to characterize electrode response while the sensing cable is still deployed in the water. It is also desirable to be able to monitor motion of electromagnetic sensor cable in the water in order to be able to determine components of the detected and recorded signals that result from electromagnetic induction caused by motion of the sensor cable in the water.